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You'll need a steady hand, but thanks to the physics of circular motion, it's really not as hard as it looks.

Bend an ordinary wire coat hanger into a diamond shape (the kind you get from the dry cleaners works just fine). The tip of the hook should be free of burs. If it is burred, file it flat or simply find another coat hanger.

Bend the hook so that the tip is level when the coat hanger is hanging from your finger.

Balance a coin on the tip of the coat hanger. This takes a bit of practice (or in my case, heaps) but it's much easier if you rest the coat hanger on a table and the coin is surprisingly stable once balanced.

Hint: 20-cent coins are much easier to balance than smaller coins.em>

Lift the coat hanger with the coin balanced on the tip. Let it swing naturally and avoid jerky movements. When you have gathered the courage, start your first full swing by moving your finger in a circular motion. The secret is to swing the coat hanger so that it always pulls against your finger.

Once you're in the swing of things, it's quite easy to keep it whirling with the coin in place. Your finger hardly needs to move. Stopping, however, is the next major hurdle.

To stop, the trick is to, again, keep the coat hanger moving so the hook always pulls against your finger even as you slow down. To do that, move your finger with a circular motion and bend your knees a little as you bring the whirling coat hanger to a halt.

Hint: as long as the hook pulls against your finger, the coin stays put!

At this stage, your audience might suspect foul play. Perhaps the coin is magnetic or maybe there was some super-glue lurking on the coat hanger? For a nifty and convincing finale, gently jiggle the coat hanger and let the coin fall into your other hand. You are a physics ninja!

What's going on?

The force that keeps the coin stuck to the coat hanger is called the centripetal force. Some explanation is obviously in order, but no amount of words can describe the Zen-like self-satisfaction of actually pulling this off.

The whirling coin is actually one of the oldest tricks in every physics teacher's book and the remarkable stability of the coin is no great mystery. High school physics students learn the 'secret' science behind this nifty demo before moving on to the genuinely weird stuff (it's called quantum mechanics, and that really is mysterious.)

But you don't need a physics degree to appreciate what's going on here because you feel the same 'force' every time you go around a corner inside a moving car. The coin is pressed against the tip of the whirling coat hanger for the same reason.

It all boils down to Sir Isaac Newton's laws of motion. Those three famous statements form the basis for all classical mechanics and they're all you need to understand this nifty bit of parlour physics. Now, please don't panic. There's no test at the end, and you already mastered Newton's laws when you were a toddler (yes, you're a genius.)

Newton's first law simply describes the obvious fact that every moving object continues moving in a straight line unless some external force acts on it. Moving in a circle feels like a different ball game, but it's that basic rule of nature that makes this trick work.

If you're standing on the roof of a car driving along a straight road that bends to the right, your body will keep moving in a straight line. If the bend is too sharp, you'll slide straight off the roof, which makes 'car surfing' one of the dumbest things you could ever do.

As a passenger inside the car, your body still tries to keep moving in that straight line too, but the door on your left stops you falling out. It 'feels' like the door is pressing against you, but it's actually you pressing against the door. Lose the door and seat belt and you'd fly out of the car, but none of this is news to you or anyone who's ever been for a drive. We're all very familiar with this sensation.

Now, if you flip the road so the corner turns up in a vertical plane instead of turning to the right, the same law of motion still applies. The difference is that gravity pulls everything down so if you stop moving when you're upside down, you'll fall to the ground.

To stay put when the coat hanger is upside down, the coin needs to press against the tip with a force that is greater than the downward force of gravity. Surprisingly, the coat hanger doesn't need to move very fast for that to happen.

The magnitude of the centripetal force is directly proportional to the mass and speed of an object (actually, the square of the speed). It is inversely proportional, however, to the radius of the circular path. So the faster the speed of rotation or the smaller the radius, the greater the force.

Because the coin is whirling around in a circle with a radius of about 40 centimetres, it only needs to go just over seven kilometres per hour. That's slightly faster than a brisk walk. A toddler could probably outrun it!

But what if you wanted to take the coin's place and loop the loop on a gigantic coat hanger? You'd obviously need to go a bit faster, but how much? Let's say the tip of the coat hanger was 10 metres from the giant finger spinning it (the height of a typical timber telegraph pole). The radius of the circular motion would be 1000 centimetres instead of 40 centimetres, but here's the surprising thing. You wouldn't have to go 25 times faster (1000 divided by 40) than the coin because centripetal acceleration is directly proportional to the speed squared.

To outpace the force of gravity pulling you to the ground, you'd only have to go 36 kilometres per hour. You could do that in a school zone without setting off the speed camera (all you need is a huge coat hanger and a giant.)

Knowing the physics is all very nice, but even when you know the equations, have done the calculations, and asked a friend to confirm your results, this trick still feels like magic. Better yet, it'll only cost you 20 cents and with the silly season just around the corner, who could ask for more? But don't take my word for it… have a crack. And have a cracker Christmas!

Maths rulesIn case you're really interested, the force of gravity pulls everything down with an acceleration of 9.8 metres per second per second. The centripetal force is the result of centripetal acceleration which you can calculate easily with this basic equation: centripetal acceleration = velocity squared divided by radius or (v means velocity and v^2 means velocity squared):

a = (v^2) / r.

To stay pressed against the coat hanger, the force of gravity needs to be counterbalanced by the force due to centripetal acceleration. You can write that down like this:

force due to gravity = force due to centripetal acceleration

Force is mass times acceleration (the asterisk means multiplied by), so you can write:

m * g = m * (v^2) / r

And now you can cancel the m (which stands for mass) on both sides to get: